The racing bicycle has evolved into a technologically sophisticated device constructed to give the racer every possible edge over his or her competitors. In choosing a bicycle, the racer looks at, among many things, every detail of the construction of the frameset, such as the weight, aerodynamics, material, geometry and wheelbase. An important characteristic of the geometry of the frameset is a short chainstay length. In addition to the obvious role of a short chainstay length in providing the desired short wheelbase, short chainstays ensure maximum stiffness in the rear triangle, all other things being equal. The racer wants this maximum stiffness to minimize the loss of the energy exerted in pedalling to bending the frame and hence maximize the efficiency of the transfer of energy to the rear wheel.
Bicycle builders have heretofore fulfilled the racer's need for a short chainstay length in one of the following ways: (1) form a crimp in the rear surface of the seat tube where the rear wheel is closest; (2) provide a bifurcated seat tube, something like the front fork, or two separate seat tubes that straddle the rear wheel; and (3) curve the seat tube. These designs have various disadvantages. Crimping the seat tube weakens it, thus partly or wholly offsetting the advantage of the short chainstay length in making the frame as stiff as possible. Bifurcated or dual seat tubes are weak in torsion, so again the frame has reduced stiffness, and there are usually weight and aerodynamic drag penalties. Bending a tube weakens it somewhat, but this is probably the preferred solution. It has not, however, been widely accepted, probably for aesthetic reasons--it doesn't look as strong.